The present invention relates in general to microcomputer terminal systems, and more particularly to a microcomputer terminal system having a list mode of operation for the video refresh circuit.
In the patent to Lovercheck et al., U.S. Pat. No. 3,973,244, issued on Aug. 3, 1976, for a Microcomputer Terminal System, there is disclosed a microcomputer terminal system having a video refresh circuit. The microprocessor writes character and control information into the video refresh segment of the main memory. Video circuits repeatedly read this information out of the memory devices to determine the location of the character to be displayed on the video screen and to enter a composite video signal on the video screen.
It has been known that the main memory of a terminal for microcomputers included programs for execution by the microprocessor, character data and control data. The control data designates the location of the character data in the memory, the location on the video screen for displaying the character data and the visual effects or video attributes of the displayable character data. The video circuits read the control data in the main memory and caused the display of the character data on the video screen with the designated visual effects or video attributes. This operation was performed each time the video display was refreshed.
In the Zentec Corporation Microcomputer Terminal System 9003, the main memory stored in contiguous locations display data. The data portion of the video screen thereof included 1920 characters and the data storage of the main memory stored 2 screens of characters. At the beginning of each refresh cycle, the video circuits read an address pointer in the memory, which indicated the starting line number within the 2 screen area of the memory. The address pointer designated the start address of the 1920 characters to be displayed contiguously on the video screen. The Terminal System 9003 required the video attribute codes to be included in the display data at the position they were to take effect on the video screen. The position on the video screen equated to the attribute position in the data was displayed as a single character blank.
The Terminal System 9003 video screen required 1920 locations in the main memory, although less than 1920 characters were being displayed. The video attribute changes required a single character blank space at the location of each change on the video screen. The video areas of the main memory were restricted to two screens.
The Terminal System 9003 with background attributes enabled video attribute changes without taking a space on the screen. The displayable video area of the main memory was equivalent to the first screen area of the Terminal System 9003 without background attributes and occupies 1920 contiguous locations in the main memory. The second screen area of the main memory was used solely for video attribute codes. The relative location of any video attribute in the second screen area of the main memory was the same as the relative location of its associated displayable data in the first screen area. Thus, for each displayable character there was an equivalent video attribute code. On each refresh cycle of the video screen, the data characters were presented to the character generator to obtain the appropriate dot matrix on the video screen, and the video attribute codes were presented parallel to the video attribute control circuits. Hence, a video screen required 3840 locations in the main memory. 1920 were required for the displayable data and 1920 for the attributes. These locations were required in the main memory whether or not a fewer number of characters were actually displayed on the screen and whether or not a lesser number of attribute changes were made.
Heretofore, video display apparatus employed a link-list, data-encoding system for refreshing a video display. Each block of data stored in a terminal memory included a two-character link that points to successive data blocks or the memory address of the next character to be retrieved. Such apparatus has been disclosed in the patent to Waitman et al., U.S. Pat. No. 3,972,026 issued on July 27, 1976, for Linked List Encoding Method And Control Apparatus For Refreshing A Cathode Ray Tube Display, and the patent to Lyman et al., U.S. Pat. No. 4,047,248, issued on Sept. 6, 1977, for Linked List Data Encoding Method And Control Apparatus For A Visual Display.
In the patent to Koster, U.S. Pat. No. Re. 28,238, reissued on Nov. 12, 1974, there is disclosed a control and display apparatus for a digital data processing system. A table link-up operation is initiated to determine the starting address for a page to be displayed. Characters are formed by display vectoring.
The patent to Hogal et al., U.S. Pat. No. 3,886,585, issued on Dec. 7, 1976, for Video Generator Circuit For A Dynamic Digital Television Display, discloses a video generator circuit and video refresh circuits. Data is stored in the refresh buffer in 16 bits per slot. Each slot has a pointer field that contains the address of another slot. A group of slots are threaded together into a list. The lists are accessed by a table of pointers.
Other patents of interest are:
Dumstorff et al. U.S. Pat. No. 3,789,367; Boyd, U.S. Pat. No. 3,744,033; Cuccio, U.S. Pat. No. 3,543,244.